In water and sediments of receiving rivers and lakes

The total loading of the watercourses within and downstream the city of Falun with metals emanating from the mine water (untreated until 1987) and the combined mining waste deposits in the Falun area is shown in Table 4.13 as concentrations of metals in the affected watercourses (rivers and lakes). Data are given from measurements prior to 1978 and as averages for the 1990s. In the 1970s, the pH of the outflowing mine water was 2.1-3.1, while pH values between 3.6 and 6.2 usually were reported from the Faluan River. During the 1990s, i.e. the period after the start of the mine water treatment unit, the maximum copper concentration in the Faluan river was 300 (o,g/l and in the bottom water of Lake Runn, 60 (o,g/l.

Average total concentrations of copper and zinc in the water of River Dalalven, upstream of Lake Runn's outflow into the river are about 0.5 ^g Cu/l and about 2 ^g Zn/l. As will be seen in Table 4.13, the elevation of the concentrations of copper and zinc in the lower reaches of River Faluan, compared to these natural, regional background levels, would thus be about 140 times for copper and about 1,000 times for zinc, resulting from the natural mineralisation and man's exploitation of the ore body in Falun.

The table also shows that the measures taken in the late 1980s to protect the environment, such as the treatment of the mine water starting in 1987, has resulted in a certain decrease in the average metal concentrations in lower River Faluan: a 3^-fold reduction in copper and a 2-fold reduction in zinc concentrations. However, in the relatively large Lake Runn, the water was still in the 1990s highly contaminated with copper and zinc. Also in River Dalalven, there is a clear impact of the old activities in the Falun area, e.g. shown as a 2-fold elevation in total copper concentrations and a 5-fold elevation in zinc concentrations when the river passes the Falun area (Lindeström, 2003).

Table 4.13. Metal concentrations in the mine water pumped from the Falun Copper Mine and in the receiving rivers and lakes prior to 1978 and averages for the 1990s. All concentrations are total concentrations in the water, expressed as ^g/l. After Lindestrom, 2003.

Sampling point

Period

Flow 1/s

Copper

Lead

Zinc

Mine water

Before 1978

6.3

60 x 103

--

2 x 106

Faluan River, below

Before 1978

6 x 103

250

--

4,100

Faluan, above city

1990s

3 x 103

6.2

0.23

15

Faluan, below city

1990s

6 x 103

70

2.2

2,000

Lake Runn, surface

1990s

--

8.3

0.26

190

Lake Runn, bottom

1990s

--

14

0.36

410

River Dalalven below Lake Runn

1990s

0.32 x 106

1.2

0.22

20

In contrast to copper and zinc, the average lead concentration in River Faluân is only about 10 times higher than in surrounding streams. Thus, much less lead has been released from the mining wastes compared with the other metals. Moreover, lead is effectively immobilized in the bottom of Lake Runn, probably as an effect of co-precipitation with iron, which flocculates and settles in great amounts in the lake. Iron was one important constituent of the ore exploited in the Falun Mine. The calculated degree of immobilization of metals in Lake Runn, expressed as an average for the years 1990-97, was for copper - 45%, for lead - 80%, and for zinc - 25% (Lindestrom, 1999). This means that some 75% of the annual input of zinc to Lake Runn, or 180,000 tonnes, are carried out into River Dalalven, while only 55% of the copper input (5,800 tonnes) goes the same way.

The above considerations leads to the conclusion that Lake Runn -especially its bottom sediment - acts as an effective trap of the metals released from the mine wastes in Falun. The same is true for Lake Tisken, situated just upstream of Lake Runn and being the first sedimentation basin downstream of the Falun Copper Mine. In order to give an idea about the present levels of metals in the superficial sediment layers in these two lakes, some data is summarized in Table 4.14. For comparison, the median metal concentrations in the sediment surface of 30 small lakes within the River Dalalven drainage basin are also presented.

Table 4.14. Concentrations of some metals in superficial sediments in Lakes Tisken and Runn immediately downstream of the Falun Copper Mine and median value for small lakes in the catchment of River Dalalven. Concentrations in ^g/g DM. After Lindestrom, 2003.

Lake

Copper

Lead

Zinc

Cadmium Iron

Lake Tisken

3,600

2,700

11,000

19

<300,000

Lake Runn, NW

3,800

900

12,000

29

140,000

Lake Runn, central

1,400

300

7,000

13

170,000

Lakes, Dalalven basin

30

70

350

1.8

59,000

Copper concentrations in sediments in the nearby sedimentation basins are elevated above regional background levels by 120-130 times and zinc concentrations by some 30-40 times, the latter showing about the same degree of elevation as lead. Cadmium concentrations are much less elevated. As a matter of fact, the copper level in the lake sediments, just below 0.4%, is not much different from the copper grade of the ore being extracted during the last few decades of the Mine's life-time. Even when looking at the somewhat lower levels of metal contamination in the sediments of central Lake Runn, it can be noted that these levels are still 3-15 times higher in the case of copper, and 8-25 times higher in the case of zinc than the levels observed in the most contaminated superficial sediments in the inner waterways of the city of Stockholm.

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